This invention relates to the art of producing extrusion dies for extruding thin-walled honeycomb structures, such as those that would have utility as a catalyst substrate or as a diesel particulate filter. Such honeycomb articles may be formed from extrudable material such as particulate ceramic and/or metal batch which may be sintered, and similar materials which have the property of being able to flow or plastically deform during extrusion, while being able to become sufficiently rigid immediately thereafter so as to maintain their structural integrity. In the past, conventional extrusion dies usually consisted of a metal plate with an array of feed holes formed in one face thereof which do not extend entirely through the die, and an array of thin intersecting slots formed in an opposite face of the die which are only deep enough to intersect the feed holes. The batch material to be extruded is forced under pressure into the feedholes and on into and through the interconnected slot array so as to exit the die as a honeycomb structure defined by the slots.
In many of the applications for which the honeycomb structures are intended, such as a catalyst substrate, there are many advantages in making the walls of the honeycomb article as thin as possible. The utilization of thinner walls within the honeycomb structure results in greater open frontal area, lower resistance to the flow of gases through the honeycomb structure, greater permeability, and lower loading of expensive catalysts on the surface of the walls. However, in order to make thinner walls or webs within the extruded honeycomb structure, it is necessary to make thinner discharge slots. Currently, the thinnest slots are made by the wire electrical discharge machining process (wire EDM process). A major problem exists, however, as this technology is extended to thinner and thinner slots, since it becomes difficult to control the cutting wire for large spans, and accordingly the length of a slot that can be accurately and uniformly cut is limited.
That is, during the wire EDM cutting operation, a force is exerted on the wire, with the tendency of the wire to bow, resulting in deeper cuts adjacent the edges of the die as opposed to central portions thereof. Although tension is applied to the wire, there is a limit to which such tension can be applied due to the structural integrity of the thin wire.
The prior art has set forth various forms of multi-part extrusion die apparatus, however, such art neither suggests the structural concept of a multi-part die in which each part having a discharge face contains a plurality of discharge slots and feed holes formed within the body thereof, nor the slot limitation reason for forming such multiple die parts.
U.S. Pat. No. 3,038,201 to Harkenrider relates to a generally complex multiple tube or core construction of an extrusion apparatus, wherein spaces between the outer edges of adjacent cores form both the feed holes and the discharge slots.
U.S. Pat. No. 3,790,654 to Bagley discloses an extrusion apparatus for forming thin-walled honeycombed structures wherein a central die is surrounded by an adjustable skin-forming slot. Alternatively, slots could be formed partially in adjacent plates.
U.S. Pat. No. 4,118,456 to Blanding et al. relates to an extrusion die formed of two body members, with the first body member having a plurality of large feed holes, and the second body member having a plurality of smaller feed holes communicating between the large feed holes and discharge slots.
U.S. Pat. No. 4,354,820 to Yamamoto et al. relates to a two-part extrusion die for forming honeycomb structures wherein feed holes and pooling slots are formed in one part of the die, and discharge slots are formed in the other.
U.S. Pat. No. 4,465,652 to Lentz relates to a laminated extrusion die blade support wherein the laminated plates have feed holes and discharge slots formed between the plates, rather than within the body of such plates.
U.S. Pat. No. 4,743,191 to Chao relates to a multi-piece die for forming honeycomb structures wherein feed holes are formed in a top die, but discharge slots are formed between the two parts of the die rather than within the die body parts per se.
U.S. Pat. No. 4,877,766 to Frost relates to a die for forming a mini-monolith substrate wherein both slots and feed holes are formed between telescoping tubes, rather than being formed within the body parts per se.
U.S. Pat. No. 4,984,487 to Beckmeyer discloses a die having feed holes which are within tubes, but discharge slots are formed between individual teeth blocks, and are not formed within a die body.
It thus has been an object of the invention to produce large commercial extrusion dies with uniform thin discharge slots, by first forming uniform thin slots within die parts of limited size, and then joining such parts together to form a multipart extrusion die.
A further object of the invention has been to produce an extrusion die with different cell densities or slot widths in various sections of the die, utilizing the multi-part technology of the present invention to produce unique flow characteristics within the die structure.